Incredibly, 20 to 40 million sperm are present in each ejaculation of semen. The quality of those sperm is essential for fertilisation to occur, and only the single fittest and luckiest sperm will actually get to fertilise an egg. It is the ultimate lottery.
There has been a slow but steady deterioration in the quality of sperm produced by men living in developed countries for over thirty years, leading to sperm abnormalities amongst the population. This is particularly noticeable in men who have dirty jobs, particularly in mining and farming, where they are more likely to come into contact with potent chemicals. Other workers showing adverse effects include electricians, mechanics, welders, painters and plumbers. Office workers have sperm that have only half the number of defects as those in industrial occupations. The number of sperm defects has increased by 50% between 1970 and 2000.
Overall more than a quarter of all men have some semen defects. These are not necessarily severe enough to render the man infertile, but it may delay their ability to father a child by a few months.
The number of men whose sperm abnormalities are bad enough to render a couple infertile is also rising, and the cause is not well understood, but is almost certainly environmental in some way.
It is now believed that some sperm abnormalities can be inherited, and magnified down the generations so that the abnormalities that may have been present in the grandfather are more significant in the father, and even worse in the son.
Contrary to popular science beliefs, the habits of men in previous generations can also affect subsequent generations, and particular a man’s fertility. If a man in 1900 was a heavy smoker or was exposed to industrial chemicals, these factors may not have damaged just that man, but also the Y chromosome in the germ cells responsible for producing sperm in the testes. The damaged Y chromosome is passed to the next generation, and if there is yet more damage caused by the son following the same or similar trade as his father, the damage to the Y chromosome may accumulate, so that by the third generation the damage is so significant that the Y chromosome is directing the production of damaged sperm.
The Y chromosome is affected far more than any other chromosome in the body because every other chromosome is paired with a chromosome from the other parent, and most abnormalities are compensated for by the paired chromosomes. The Y chromosome in men is not directly paired with the far larger X chromosome that comes from the mother.
This trend is becoming worrying, because if it continues, future generations may have significant problems reproducing as the quality of sperm steadily deteriorates. By using IVF we are actually bypassing the natural selection process. In previous generations, infertile men would not be able to pass on their infertility in their genes, but by using IVF techniques we may be not only perpetuating fertility and Y chromosome problems in future generations, we may actually be worsening the problem. What other genetic defects are being perpetuated by this technique are completely unknown.